1. Field of the Invention
The present disclosure relates to feed-through elements in general, but in particular those for ignition devices such as are used to ignite a pyrotechnic person protection device. In particular, the present disclosure relates to the configuration of the base of such an ignition device.
2. Description of Related Art
In particular, airbags and seatbelt tensioners are used as person protection devices in motor vehicles. Such safety systems can significantly reduce the risk of injury. A prerequisite, however, is that the safety systems in question do not fail in the event of a collision. Particular attention is paid to the igniters of such pyrotechnic apparatus, which are indispensable for the function of such safety apparatus. In particular, the igniters must still function properly even many years after their production. As an average lifetime of such igniters, 15 years is often specified. In order to guarantee proper long-term function, it is necessary to ensure that the propellant charge provided in the igniter is not degraded in the course of time. Such degradation may, for example, be caused by moisture entering the igniter. It is therefore important to hermetically encapsulate the propellant charge of the igniter. The igniter must also release the gases of the ignited propellant charge in the correct direction, in order to ignite the propellant charge of a gas generator of the safety system.
In order to ensure this, igniters known from the prior art comprise a cap or cover and a comparatively solid base, between which the propellant charge is enclosed in a cavity formed by these parts. The current for igniting the propellant charge is delivered through the base by means of electrical connections. The base therefore generally comprises access openings, in which there are metal rods that can be supplied with the electrical current on one side by means of a plug connection and are connected on the other side, for example by means of an ignition bridge which causes the propellant to ignite when the flow of current comes in contact with the latter. The base is therefore generally referred to as a feed-through element. When configuring the feed-through element, it is necessary to ensure that when the propellant charge is ignited, the cap or cover or a part of it always breaks and the electrical feed-throughs are not driven out of the base.
Two technologies have gained acceptance on the market for such feed-through elements. In the first, the support body of the base consists of metal and the ignition bridge is produced by means of a bridging wire welded on. In this embodiment, a metal rod is fixed as a pin in an electrically insulating fixing material in an access opening of the support body. A glass material, in particular a resin glass or glass solder, is conventionally used as the glass material. This metal rod is therefore insulated from the outer conductor by glass. A second metal rod as a pin is welded or soldered to the outer conductor which is represented by the support element, also known as a baseplate. On the inner side of the feed-through element—that is to say the side which faces towards the ignition cap of the finally mounted ignition device—a bridging wire (usually made of a tungsten alloy) as an ignition bridge comes in contact with the surface of the glass material. So that the bridging wire is not damaged and the ignition element has a long lifetime in service, for example in a motor vehicle, the surface of the glass material must be ground since surface roughness can damage the bridging wire.
The length of the wire influences the resistance and therefore the triggering characteristic of the ignition device. In the event of ignition, the resulting explosive pressure acts on a small glass surface, so that this embodiment may be regarded as very robust. Another acknowledged advantage of this version is that a pin is directly connected to the outer conductor, and simple earthing of the igniter takes place through this pin. Disadvantages are the higher process costs due to the surface grinding of the glass material. Furthermore, only stainless steel can be used for the outer conductor for corrosion reasons, and the resistance depends on the positioning tolerance of the pin in the glass as a fixing material and on the wire length. This type of igniter is nevertheless the most widespread one.
Ignition devices of this type are known, for example, from DE 101 33 223 A1. The version described in US 2003/0192446 A1 also belongs to this group, even though grinding can be obviated therein since the plane surface, on which the bridging wire comes to bear, is produced by an additional ceramic body. This, however, entails extra production costs. Furthermore, the pin which is intended to establish the connection to the outer conductor is covered by the glass material. This prevents visual inspection and therefore makes the required quality inspection during production more difficult.
A second technology used in order to produce ignition devices is based on support bodies made of pressed glass as a base, through which two metal rods are fed as an electrical supply and connection elements. A ceramic with a thick-film conductor as an ignition bridge is soldered onto the pin ends. Two short pin ends on the inner side extend beyond that of the base, i.e. they have a projection relative to the glass surface. In order to produce such a feed-through element, the liquid glass must be elaborately pressed. Since both pins have been insulated, a connection to the outer conductor must be established. This is done as described in EP 1061325 A1 by means of an additional component. The advantages of this embodiment are the freer selection of the outer conductor material, and the fact that the positioning tolerances of the pin in the access opening do not affect the resistance since it is predefined by the ceramic substrate or chip. Disadvantages are the larger area of glass, which weakens the design, as well as the more elaborate earthing and higher total costs of the system. This type of igniter is therefore less widespread.
Owing to the described stability requirements of the base, its support body has to date been configured very solidly. This requires the outer contour of the support body to be formed by turning on a lathe, whereas the access openings have been drilled. Both processes are time-consuming and therefore make production more expensive.
U.S. Pat. No. 6,557,474 B1 proposes to configure the support body as a stamped metal part. The fundamental problem with stamping support bodies, however, is that the access openings have to be stamped with great accuracy, particularly in respect of the diameter variance and the profile of the access opening. The thicker the support body is, i.e. the higher its material width is, the greater the inaccuracies are. U.S. Pat. No. 6,557,474 B1 is therefore based on a very thin support body, which is in conflict with the requirement for stability of the component. In this document, a relatively thick glass layer is therefore applied onto the stamped metal part in order to stabilize it.
However, the glass must still be pressed. In the event of the ignition, the entire explosive force acts on the glass, and it is therefore not mechanically stable enough. In a structure of this type, the connection between the glass and metal can only be made by means of a chemical reaction, to which end the glass and the metal must have the same thermal expansion. This is possible solely and exclusively with an NiFeCo alloy as the material for the stamped metal part. The material costs of the NiFeCo alloy, however, are extremely high. Owing to these disadvantages, this embodiment has not yet come into use.
EP 1455160 B1 proposes to use a single stamped metal part of sufficient stability as the support body. Both the outer contour of the support body and the access opening, in which a pin is fixed by means of a glass solder, are formed by a stamping process. The pin, which establishes the contact with the outer conductor, is not fixed in an access opening in this embodiment but instead soldered over a large area to the lower side of the support body. Stamping of the access opening, in which the glass-metal fixing takes place, is possible since the access opening is subject to minor requirements with respect to the accuracy of the diameter and the profile, since with suitable process management it is possible to compensate for large solder gaps and therefore also large tolerances by fixing the pin with the glass fixing material. Conventionally, the upper side of the glass surface is ground, so that this embodiment belongs to the group of feed-through elements mentioned first. This embodiment also suffers from the disadvantage that the support body conventionally consists of a stainless steel, because otherwise the support body made of a non-stainless metal would need to be coated in order to avoid corrosion. In the case of such coated support bodies, however, the glass surface of the glass-metal feed-through can then no longer be ground because otherwise the coating would be abraded as well. The costs for production furthermore are increased by grinding and polishing the glass surface of the one glass-metal feed-through, by welding the bridging wire and the process outlay for producing the large-area soldering of the earth pin onto the support body.
Because welding the cap to a stamped support body can lead to thermal stresses of the likewise stamped glass-metal feed-through, which may put its leaktightness at risk, DE 10 2005 009 644 A1 proposes to provide the support body with a thin welding edge. This document discloses embodiments with an access opening and an earth pin, which is soldered to the lower side of the support body in the manner of the aforementioned EP 1455160 B1. An alternative embodiment presents a support body with a stamped and drilled access opening, and an ignition bridge applied as a thick-film conductor.
Against this background, it is an object of the present disclosure to provide a feed-through element which is suitable for use in pyrotechnic person protection devices, but which is produced at reduced costs.